Unfortunately, after modifying the sample code to my needs, a SOAP exception kept getting thrown: "NodeType 'Element' is not an valid type":

<errorstring xmlns="http://schemas.microsoft.com/sharepoint/soap/">'Element' is an invalid XmlNodeType. Line 1, position 10.</errorstring>

Using Visual Studio 2005 to debug, I have been unable to work out what the problem was. The CAML query built in the webservice to create the field must be incorrect, but the innerXML of the CAML query (I was only using the "updateFields" node) looked fine. Clearly the problem was elsewhere in the call, but is not clear to me how the call is constructed once the parameters are passed to the web service.

SOAP does not seem to me to give a huge amount of debugging information when a call fails. When I create my own web services, I can use VS to step through them and get lots of debug information, but I haven't been able to work out how to do this with the MS DLLs yet.

Anyway, I eventually solved this problem by going back to the example and creating another copy of the example Microsoft code with minimal changes in order to point the call at my webservice. This proved to work OK, although it did create two sample fields (Field1 and Field2) that I cannot remove using the Sharepoint IE front end!. I suspect this is becuase they are created as required fields.

By making minimal iterative changes, I finally managed to create the following c# method which successfully creates the fields on the fly, and even gives them an appropriate Sharepoint type.

(Please note this code looks better in IE than Firefox which is not wrapping the lines - click on "View Plain" to view or copy in Firefox)